Add support for RDoc 4

This is taken from Atsushi Nagase's changes in his hanna-bootstrap
fork, commit f8f5968c14e0635546bc27ab835296bc6a3c063e.
Bump version to 0.3.0, update RDoc dependency to ~4.0

This contains some other fixes to the gemspec so that current
rubygems will actually build a gem from it.
Show superclass instead of parent for classes (Fixes #5)

Parent is the enclosing module/class, and in general is not very
useful.  Superclass is much more useful, so show that instead.

Most of this patch is from Quintus, I only fixed the patch and
changed the label to Superclass.
Remove test, rcov, and default tasks

There are no tests, so having rake tasks for them makes no sense.
Fix Gemfile, remove Gemfile.lock

For development, there is no need to lock dependency versions.

In the Gemfile, reference haml, fix rdoc version, and don't use
a grouping since the Gemfile is only for development anyway.
Remove use of jeweler

Jeweler really doesn't save you anything compared to writing the
gemspec manually, and was generating a bad gemspec, so just remove
the use of jeweler.

Also, remove the use of bundler in the Rakefile, since there
doesn't appear to be a need for it.
Get rdoc rake task working

Require rdoc/task as rake/rdoctask was removed in Rake 10.  Use
hanna itself when for the rdoc, as dogfooding is good.

As rdoc didn't actually generate useful output probably due to
encoding issues, replace non-ascii characters in main file so
that it actually runs.

Add a clean task that cleans the rdoc directory and any generated
.gem files, and change the gitignore file to ignore rdoc instead
of doc.